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Principles of medical imaging I
Lecture Details Marilyn Baird; Week 2 MED1022; Anatomy Lecture Content X rays are short wavelength, are produced in a rotating anode x-ray tube. The cathode emits electrons which are attracted across the gap to the anode, electrons then hit the anode and are drastically decelerated. This deceleration produces x-ray photons. An x-ray contains a head containing the x-ray tube and an x-ray collimator for defining the x-ray field. There is also a control panel to select the kilovoltage and tube current as well as an exam couch. Kilovoltage represents the penetrative power of x-rays. Millamperage represents the current flowing through the x-ray tube. mA is the product of the time in seconds or less and the mA. One of 3 things can happen to any given x-ray photon passing through the patient. It can a) pass through unaffected; b) be annihilated; c) be scattered from its original direction. Absorption and scattering can cause the reduction of the x-ray beam through the patient- attenuation. Whatever remnant beam is left is captured by one of three devices to produce the radiograph. Devices to record these can be a) film screen cassettes (no longer used) which sandwich an x ray film between 2 intensifying screens which are housed in a radiographic light tight cassette. The cassette is then taken to a dark room and the film is processed using chemicals. b) computed radiographic systems which have cassettes with photostimulable phosphors which trap x-ray energy, leaving the patient with the latent image to be reconstructed. CR reader removes the plate and scans it with a red laser. The impinging laser photons cause the trapped electrons storing the latent image to be released and to release blue-green light. A photomultiplier tube detects the light and the amount of light emitted is correlated by the computer and an image is prodduced. c) Digital radiographic systems is when the plate becomes an electronic flat panel which is divided into image areas corresponding to a pixel. It is connected to the computer by a cable. Can be indirrect which uses a scintillation materioal, or direct which uses a photoconductor. X rays can be dangerous as the x-ray photon meets the atom and electrons may be ejected or raised to a higher energy state within the atom. Ionised atoms have altered bonding properties. Ionisation is reversible and patients need prolonged exposure for harm. X rays can kill cancer cells and promote their growth, cells that comprise tissue have different sensitivities. Rapidly dividing cells have high sensitivity, muscle and nerve low sensitivity. ALARA is the radiation protection principle- as low as reasonably achievable. Justification- benefit must outweigh the risk; optimisation is where the image must be optimised within lowest possible dose; time should be as short as possible; distance should be as large as possible; shielding should be used where appropriate. Collimaton is act of restricting x ray beam to area of interest to reduce radiation hazard, reduce scatter radiation. Radiation protection should be used in areas beyond the ROI. Gonadal shielding and pregnancy should be checked for. Radiograph is an image of anatomy using radiographic equipment, radiographic projection is positioning term describing path of the beam, its entrance and exit points. Radiographic position is the position assumed by the patient. Principles of imaging- minimum of two projections at right angles (AP/PA and lateral) to overcome superimposition of anatomical structures, localise and locate lesions and foreign bodies, determine alignment of #s, confirmation of pathology. Exception to the right angle principle is when 3 projections are required when examining joints. The greater the tissue density, the greater the attenuation of x-rays. Many soft tissues cannot be visualised without contrast. Air is generally dark, radiolucent. Bones are generally white, radioopaque. Elbow is formed by coronoid and radial fat pads. Supinator fat stripe is anterior to the proximal radius and exposed later with a radiograph. Radiological interfaces is the extent to which you can see structures that lie adjacent to each other, depends upon their position relative to the path of the x-ray. Lines on radiographs can be seen when their interfaces are parallel to the path of the x-ray. Distortion and beam divergence can be minimised by changing source image distance, object image receptor distance, central alignment/centering. Increase in optical density due to superimposition is called summation. Myelogram is to image nerve roots. Readings